Diesels nozzles are bigger than unleaded nozzles. (diesel nozzles and the old 4star nozzles were the same size, unleaded is smaller than both of them)
I disagree that it's the garages fault at all too, if people are too busy to spend 2 seconds checking they are using the correct fuel then it's their fault, not the garages.... and if anyone doesn't know what fuel they should be putting in the vehicle they are driving then they shouldn't be driving it.
Anyhow, to the question, in an old diesel it would do no harm, in a modern diesel it will damage things.